Hey if anything the author is using her jealousy in a good way to progress the story.
Instead of the stupid trope where she gets mad at him and then there's pointless drama and miscommunication. Here she saw competition and realized she needs to step up her game to make sure he is too focused on her to even see notice other women exist.
Well he already doesn't notice other women but still, time for more cute antics!!
You're forgetting that half the good part of this manga is watching her get jealous and then seeing her get flustered when she once again realizes that he was only ever thinking about her
If she wants to keep her territory maybe she should mark it... like idk not keep it a secret. Instead she is not letting anyone know he is taken and then acting surprised that other girls are going after him.